A cooking utensil can comprise a round container for holding the food, the container having a base, a plate made of heat-conducting material and a cap enclosing the plate, the base of the enclosing cap being given a convex curvature towards the container and the cap having a collar which abuts the container in a curved transition region between the container base and the container jacket.
The container and the cap can be made of stainless steel and the plate can be intermetallically bonded to the container base and the cap.
In order to produce the intermetallic bond a plate can be inserted into the cap so as to leave an annular flow space between it and the preformed cap collar.
The assembly comprising the container base, the plate and the cap can be centered between the punch and the die of a press and intermetallically bonded by single or multiple impulse pressure so as to form the curvex curvature of the cap bottom and to mold the rim of the cap around the container. During the pulse pressure the plate is pressed by plastic deformation into the flow space, where it is intermetallically bonded to the cap base, the cap collar and the corresponding region of the container.
Of course the method can be carried out at ambient temperature or with heating.
The pulse energy will be adjusted to the temperature of the container, at least in the base region, the cap and the plate. Usually these temperatures are chosen so as to be near but sufficiently below the melting point of the plate material.
Aluminum in the form of industrially pure aluminum is a very useful plate material. Copper is also a useful plate material.
The cooking utensils can be pots, pans or the like, more particularly cooking pots and pressure cookers.
The plate improves the heat transfer from a hotplate or stove plate to the food. The heat transfer is adversely affected if the intermetallic bond is damaged. This applies even if the damage occurs in the region of the cap collar.
Of course, the tools, i.e. the die and the ram, for applying pulse pressure in a press, must be adjusted so as to obtain the aforementioned slightly convex shaping of the cap bottom. Usually the container base is made slightly convex outwardly. The geometrical conditions with regard to the container base, the cap and the plate must be adjusted so that in the finished cooking utensil, the plate completely fills the space between the container base and the cap, i.e. enters and fills the flow space when the intermetallic bond is manufactured.
A defined, undisturbed flow of plate material is important for the intermetallic bond. The plate can be a flat plate or can have a lenticular or stepped cross-section. Optionally according to the invention, the surface of the cap and the container base facing the plate are modified so as to assist the intermetallic bonding, e.g. by etching, roughening or the like.
It is particularly important also that the rim of the cap collar should faultlessly abut the container. if plate material flows out here, the container will have to be rejected for aesthetic reasons and to avoid corrosion.
If there is a gap, water can enter during use or washing and can cause trouble through corrosion.
In the known method, on which the invention is based (EP 0 209 745), the operation is as follows: the plate is secured in the center of the cap surface by spot welding. The assembly comprising the cap, plate and container base or container is heated to a temperature which is near but below the melting point of the plate material. The initial thickness of the plate is at least 20% greater than the final thickness of the plate after the intermetallic bond has been formed.
In a first phase of pulse pressure, the pressure is applied progressively from the center to the edge of the assembly so as to obtain a special convexity, i.e. so that the total curvature of the convexity of the shaped plate and of the container is not less than 0.5% of the average diameter of the container base in its final shape.
This method drives air from the center outwards from the assembly comprising the container base, plate and cap.
The known method, however, results in constraints which restrict the usefulness of the previously-described steps. This is because of certain conditions: on the one hand the initial thickness of the plate must be at least 20% greater than the final thickness thereof after manufacture of the intermetallic bond, and on the other hand the convexity must be adjusted as previously described.
Also, it is not possible under all conditions to ensure that the spot-welded connection between the plate and the base of the cap is resistant to pulse pressure. If it tears during application of the pulse pressure, displacements during the pressure pulse cannot be avoided, and consequently it is impossible to ensure that the plate is given an intermetallic bond which meets all requirements even in the edge region and the cap collar.